Extended Abstract (76K)P1.8
Orographic Influences on Rainfall and Track Associated with the Passage of Tropical Cyclones
Yuh-Lang Lin, North Carolina State University, Raleigh, NC; and D. B. Ensley, S. Chiao, C. M. Hill, and C. Y. Huang
In this study, we employ a mesoscale nonhydrostatic model (COAMPS) to simulate Supertyphoon Bilis and investigate the dynamics of orographic rain and track deflection affected by Taiwan's Central Mountain Range (CMR). The model performs well in terms of the prediction of Bilis? track and the orographic rainfall distribution, but the rainfall amount is overpredicted. A flux model is used to help predict the rainfall distribution by calculating distributions of both the orographic moisture flux and the vertical moisture flux. The vertical moisture flux calculated from the 15-km resolution simulation compares reasonably well with the observed rainfall distribution. Thus, the flux model based on a coarser-resolution numerical simulation may be used to help predict heavy orographic rainfall. Based on the analysis of previous studies of tropical cyclones passing over Taiwan's CMR, we propose that the non-dimensional parameters Vmax/Nh and Vmax/U may serve as control parameters for determining track continuity. An induction mechanism is proposed to help explain the northward movement of a cyclone located to the southeast of an idealized Taiwan topography in a quiescent atmosphere. The northward deviation induced by the cyclonic circulation associated with a secondary vortex generated by interaction between the parent cyclone's outer circulation and the orography. Vorticity to the southwest of the mountain range is mainly produced by the vorticity stretching, while vorticity increases over the east slope and ocean are due to vorticity advection. The upstream vorticity center moves to the north for a cyclone located to the east of the mountain in a quiescent fluid, and a secondary vorticity maximum develops on the western side of the mountain. The northward movement of the parent vortex is mainly caused by vorticity advection and slightly enhanced by vorticity tilting. For a cyclone located to the northeast of the mountains in a quiescent fluid, a banner of positive vorticity develops and wraps around the northern portion of the topography while the vorticity center moves to the north of the mountain. The formation of the vorticity banner and the cyclonic movement of the vorticity center around the northern edge of the mountain are mainly due to vorticity advection. The net movement of a tropical cyclone over a mesoscale mountain range depends on the relative strength of the basic flow deflection and the deflection of the tropical cyclone's outer circulation by the topography.
Poster Session 1, Tropical Cyclones, Large-scale Dynamics and Convection
Monday, 29 April 2002, 11:00 AM-12:30 PM
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